nixos autoUpgrade: add a 'dates' option

(cherry picked from commit 80917d2473d321d7a990cf991c6353547976638c)
Originally merged #11808 to release-15.09 directly. Close #11821.

nixos/modules/installer/tools/auto-upgrade.nix

@@ -42,6 +42,17 @@ let cfg = config.system.autoUpgrade; in
         '';
       };
 
+      dates = mkOption {
+        default = "04:40";
+        type = types.str;
+        description = ''
+          Specification (in the format described by
+          <citerefentry><refentrytitle>systemd.time</refentrytitle>
+          <manvolnum>5</manvolnum></citerefentry>) of the time at
+          which the update will occur.
+        '';
+      };
+
     };
 
   };
@@ -73,7 +84,7 @@ let cfg = config.system.autoUpgrade; in
         ${config.system.build.nixos-rebuild}/bin/nixos-rebuild switch ${toString cfg.flags}
       '';
 
-      startAt = mkIf cfg.enable "04:40";
+      startAt = optionalString cfg.enable cfg.dates;
     };
 
   };